1. Field of the Invention
The present invention relates generally to a method and an apparatus for soldering surface mounted electronic components to and desoldering them from metallic contacts on substrates such as printed circuit boards.
2. Summary of the Invention and Description of the Prior Art
A convection heat system for metallic solder attachment and detachment of components surface-mounted on printed circuit boards is disclosed. For soldering, the system conveniently, rapidly and precisely positions the workpiece component, the printed circuit board and a source of convection heat relatively to each other through either moving the printed circuit board using X-Y table or through moving the vacuum-held workpiece with the source of convection heat to a desired relation with the printed circuit board using an arm system that moves in X, Y and Z directions independently of the X-Y table. The source of convection heat used for melting the solder is hot gas which is forced to flow down the terminus of the arm inside a nozzle conduit that encompasses the component work area where the hot gas deflecting inwardly from the nozzle conduit walls moves rapidly past the area to be heated and then outward from the printed circuit board working area. At the same time that the hot gas flow is melting solder, cooling air is forced down a tube inside the nozzle conduit onto the top central portion of the workpiece component surface. The tube used for passage of the cooling air may advantageously be used also for the vacuum holding of the workpiece since the two operations take place at different times. A thermal sensor in the immediate vicinity of the solder joints effectively measures the temperature of the solder so that the heating process can be controlled or terminated.
Hot gas has been used in a number of existing systems to melt solder contacts between surface mounted components and printed circuit boards so as to release or secure these contacts. One such system was disclosed in the U.S. Pat. No. 4,426,571, where the main problem is that the hot air flow is not restricted to the immediate vicinity of the workpiece, and overheating of components adjacent to the workpiece occurs. Large amount of hot air required in this case often causes damage to the printed circuit board. In the present invention hot gas is restricted to the immediate vicinity of the workpiece by means of a nozzle conduit.
Another problem encountered in practical applications of the method disclosed in the U.S. Pat. No. 4,426,571 is damage of circuitry printed on the printed circuit board. This damage may easily occur when the workpiece is picked up by hand or a hand held tool while the solder contacts connecting the workpiece to the printed circuitry are not completely melted. The present invention provides vacuum pick-up so that no excessive force may be applied to lift the workpiece off the printed circuit board.
The idea of restricting hot gas to the immediate vicinity of the workpiece has been previously disclosed in the U.S. Pat. No. 4,552,300, where hot gas fills the cavity formed between the nozzle walls and the printed circuit board, and the workpiece is located in the cavity. This method proved to be inefficient in transferring heat from hot gas to solder contacts. In the present invention the heat exchnge occurs between hot gas rapidly moving directly past the solder contacts into the area outside the nozzle conduit.
Another problem that often occurs when soldering or desoldering contacts between the workpiece and the printed circuit board is overheating and/or thermal shock damage of the workpiece itself. The present invention provides means for cooling the workpiece and shielding it from direct incidence of hot gas to avoid overheating and thermal shock damage of the workpiece.
In systems such as CRAFT-100 by PACE Inc. and systems manufactured by AIRVAC little or no visual and physical access is provided to the workpiece during positioning of the workpiece on the printed circuit board. In the present invention convenient visual and physical access is provided to the workpiece at any time during positioning by allowing the nozzle conduit to move away from the workpiece during positioning and completely enclose the workpiece during soldering.
In existing systems, relative positioning of the workpiece and the printed circuit board is accomplished by moving the printed circuit board so that the area of interest is precisely underneath the component and the source of heat. Moving the printed circuit board, which is often many times larger than the component, in precise manner is often inconvenient. The present invention provides means for moving the workpiece held by vacuum at the end of an arm mechanism together with the source of heat as well as means for moving the printed circuit board to position the workpiece, the printed circuit board and the source of heat relatively to each other.
It is, thus, a principle object of the present invention to provide an apparatus for soldering and desoldering of contacts between surface mounted components and printed circuit boards that is more convenient and efficient than existing apparatuses.
It is an important object of this invention to provide an apparatus as described in which an efficient transfer of heat from hot gas to solder contacts is achieved by creating a turbulent rapid flow of hot gas directly past the solder contacts.
It is a further object of this invention to provide an apparatus as described where the printed circuit board and the components adjacent to the workpiece are protected from overheating by restricting the application of hot gas to the immediate vicinity of the workpiece.
Another important object is to provide an apparatus as described in which the workpiece itself is protected from overheating and thermal shock by providing means of cooling the workpiece and shielding it from direct incidence of hot gas during the process of soldering and desoldering.
It is a further object of this invention to provide an apparatus as described in which the hot gas is delivered to the solder contacts at high flow velocity without significant impact, eliminating splattering of the solder, by distributing uniformly the hot gas flow along and deflecting the flow from the walls of the nozzle conduit used to restrict hot gas application.
It is an important object of this invention to provide a method and an apparatus to position precisely and conveniently the surface mounted components, the printed circuit board and the hot gas flow relatively to each other.
It is yet another object to provide an apparatus as described above in which there is an easy and convenient visual and physical access to the workpiece and its location on the printed circuit board by allowing the nozzle conduit to move relatively to the vacuum-held workpiece.